When we first see Luna, she is sitting alone in a compartment on the Hogwarts Express; Neville says that the compartment is occupied, and Ginny overrides him, saying that it's just Looney Lovegood. Entering the compartment, Harry notes that she seems to be reading The Quibbler upside down and wearing a necklace of Butterbeer corks. Harry sees that one of the articles in The Quibbler seems to involve Sirius Black, and borrows it from her to read the article, which states that Sirius is actually one Stubby Boardman, popular musician. Quickly leafing through the paper, Harry notes that it is all similar sorts of nonsense, before he hands it back. Hermione Granger, when she arrives, speaks disparagingly of the paper, whereupon Luna frostily replies that her father is the editor, and retreats icily behind her upside-down paper.

When Harry leaves the train, he sees that the carriages he thought were horseless are apparently being pulled by vaguely horse-shaped, winged reptilian creatures. He asks Ron what they are, but Ron cannot see them. Harry is only slightly reassured when Luna tells him that she can see them also.

Harry gets in trouble with the new Defence Against the Dark Arts teacher, Dolores Umbridge, for stating that Lord Voldemort had returned. Shortly after this, Luna stops Harry outside the greenhouses, saying that she believes him. Harry wonders how much the reassurance is worth, coming as it does from someone who seems to be wearing earrings made from radishes.

Luna is one of the twenty-five students who appear for the first meeting of the Defence Association, or as it is later known, Dumbledore's Army. At the first meeting in the Hog's Head, Luna says that Fudge has his own private army, so it is not surprising that he believes Dumbledore would be creating one. Asked about this, she says that Fudge has an army of Heliopaths. Hermione quickly moves on to the business of the meeting, rather than allowing Luna's odd views to take up time.

At the final meeting of Dumbledore's Army before Christmas, Harry arrives early to find that Dobby has decorated the Room of Requirement for the season. He manages to get most of the embarrassing decorations down, but there are still some streamers and mistletoe. Luna, arriving, says that it looks very nice, but warns Harry that he shouldn't stand under the mistletoe because it is likely full of Nargles.

Since their announcement of the return of Voldemort, both Harry and Dumbledore have been repeatedly mocked in the public press, most notably the Prophet. In February, Hermione arranges for Harry to meet with Rita Skeeter, the muck-raking journalist who had pilloried Hagrid in the press the previous year. Hermione had discovered that Rita was an unregistered Animagus, and had forced her, under threat of revealing this to the Ministry, to stop writing for a year. It has occurred to Hermione that if Harry's full story gets published, it might help Harry's cause. Luna is present at this meeting because, as mentioned earlier, her father publishes The Quibbler, and that would be the venue where the interview would be published. Rita is annoyed that the Quibbler doesn't pay for editorial content, but under threat from Hermione, interviews Harry and writes the story. Luna says that she does not know when the story will appear, because her father is expecting something big about Crumple-Horned Snorkacks to come in that week.

Later, when Harry's interview appears, Luna says that her father is quite pleased. This edition of The Quibbler has sold out, and he is having to make a second print run.

Harry determines that his godfather, Sirius Black, may be being tortured at the Ministry of Magic by Voldemort, and that his only way to determine whether this is actually happening or not is to call Sirius at Grimmauld Place and see if he is there. The only unmonitored fire in the school is in Umbridge's office. Luna, who with Ginny has heard Harry shouting at Ron and Hermione, volunteers to help; Harry sets her to guard one end of the corridor by Umbridge's office, so that he can break in and talk to Sirius. Luna's efforts come to naught, and she is apprehended by one of Umbridge's Inquisitorial Squad and brought in to Umbridge's office. She stands, barely restrained and not resisting, apparently only somewhat aware of what is going on, though she does seem somewhat startled by Hermione's "revelation" of what it was that Harry was trying to do.

After Harry and Hermione have escaped Umbridge and the Centaurs in the Forbidden Forest, Luna, Ron, Ginny, and Neville meet up with them. When Ron and Harry wonder how they are going to get to the Ministry to save Sirius, Luna says that they will have to fly. Ron asks sarcastically if they will use the Snorkacks for that, and Luna says that's silly, Snorkacks can't fly, but "they" can. She points, and Harry sees two Thestrals approaching, attracted by Grawp's blood that has fallen on Harry and Hermione. Harry decides that he and Ron will head off to the Ministry, but Luna tells him that there are more Thestrals approaching, he and Hermione must really smell. Ron, Ginny, and Hermione are unable to mount the Thestrals, as they can't see them, but Luna dismounts and gets them settled on the creatures; backs, and they all take off for the Ministry.

Once at the Ministry, Luna accompanies Harry and the others as they go in search of the room where Sirius is being tortured. One room they find is a stone amphitheatre with a tattered veil hung in an archway. Harry is almost certain that he can hear people on the other side of the veil, and Luna says she hears them also. Eventually, they reach the hall of Prophecies, where they find that in fact it was a ruse, that Sirius is not there, and instead they are confronted by twelve Death Eaters. In the ensuing battle, Harry, Hermione, and Neville get separated from Luna, Ron, and Ginny. They meet in the antechamber, at which point only Luna and Harry are fully capable: Hermione has been knocked out by means of an unknown curse from Antonin Dolohov, and Dolohov also broke Neville's nose and wand, Ron has been hit by what acts rather like a drunkenness spell, and Ginny's ankle is broken. They are then forced to retreat back into the Department of Mysteries, where Luna is rendered unconscious, while trying to seal a door against the Death Eaters.

Following their return to Hogwarts, Harry is mourning Sirius' death. Meeting Luna by chance in the halls, he unburdens himself a bit to her, knowing that she has also seen someone die, because she had been able to see the Thestrals. She says that she had seen her mother die; she had been a very smart witch, and had been experimenting with new charms, when one had backfired on her. She goes on to say that she knows she will see her mother again, and that Harry will see Sirius again. Both of them had heard the voices on the other side of the veil in the room with the archway, and Luna believes that those are the dead, speaking to them. Harry somehow feels comforted by this.

We first see Luna again on the Hogwarts Express. Here, she, Neville, and Harry share a compartment. Shortly, a group of fourth-year girls arrive; their spokesperson, one Romilda Vane, introduces herself and invites Harry to come sit with them. Harry replies that he's with his friends, and will stay where he is, thanks anyway, and the girls retreat, confused. Luna remarks complacently that she is seen as being rather odd, and she knows that others call her "Loony Lovegood". She also asks if Harry plans to start up Dumbledore's Army again, and Harry says there's not much point in it. Luna says that she liked the DA, it was almost like having friends. Harry, slightly discomfited, turns the conversation to OWL scores.

Ron and Hermione have had a bit of a falling out over Ron's ongoing infatuation with Lavender Brown, and Ron manages to say something particularly hurtful to Hermione. After class, Harry finds Hermione being comforted by Luna, and returns Hermione's things; Hermione thanks him and heads off to class. Luna comments that Ron is nice enough, but he can say hurtful things sometimes. Harry thinks that Luna is sometimes rather more honest than is comfortable. Harry has been under some pressure to select a date for the Christmas party that Professor Slughorn is holding, and decides that he can ask Luna to go with him, just as friends. Luna says that would be fine, and accepts. Peeves overhears, and gleefully starts spreading the word throughout the school, as only he can.

Harry is relieved to find that Luna has left the butterbeer-cork necklace and the radish earrings off for the party, and escorts her up to Slughorn's office. There, Luna starts discussing Cornelius Fudge and his plans for world domination with the drunken Professor Trelawney, who proves a ready audience.

With the graduation of Lee Jordan, the school has been trying out new commentators for Quidditch, and much to Harry's surprise, Luna is the commentator for the Quidditch match that occurs in March, Gryffindor against Hufflepuff. Harry does not finish the match, having been hit in the head by a mis-hit Bludger, but Ron tells him later that the commentary was most entertaining.

Luna appears briefly again, carrying a message from Dumbledore. In the process of finding the message, which is notice of Harry's next private lesson, Luna finds, and presents to Ron, something looking like a large green onion which she says is a Gurdyroot, good for warding off Gulping Plimpies.

Near the end of the book, Dumbledore tells Harry that they will be going to seek a Horcrux. Before they depart, Harry gives Ron and Hermione the rest of his Felix Felicis potion, and charges them with getting any DA members who will help and aiding the defence of the school. On their return, Harry and Dumbledore find the school under attack. In the aftermath, Harry learns that Hermione and Luna had been watching Snape's office. When Professor Flitwick had gone to tell Snape that his assistance was needed, Snape had apparently Stunned Flitwick. Departing his office, he had seen Hermione and Luna and sent them to aid Flitwick, who he said had apparently fallen and injured himself. Caring for Flitwick had effectively kept Luna and Hermione out of the battle.

Luna appears with her father at Fleur and Bill Weasley's wedding, where she is ecstatic at having been bitten by a Gnome; apparently both she and her father believe it might grant her some special ability. Luna, annoyingly, recognizes Harry through his disguise (Polyjuice potion), not seeming at all surprised that he looks different, or that he is using a different name. Once the music starts, she dances by herself.

Overhearing a conversation from outside their tent shortly before Christmas, Harry learns that Luna, Ginny, and Neville had been caught trying to break into the Headmaster's office at Hogwarts and steal the Sword of Gryffindor. While we learn that she was punished for this, we can only surmise that the punishment may have been the same as was meted out to Ginny at the same time: detention, to be served with Hagrid supervising.

Having also overheard that Luna's father was running one of a very few pro-Harry news organizations, Harry, Ron and Hermione visit him just after Christmas. Xenophilius provides what hospitality he can, but Luna does not seem to be present; Xeno says that she is down past the bridge, catching Plimpies. Harry wanders upstairs and looks around her bedroom, and finds that she has decorated it with pictures of herself, Hermione, Harry, Ginny, Neville, and Ron, wound around with what looks like a golden ribbon but is the word "friends" written over and over. Harry notes that the room is apparently unoccupied and has some months worth of dust. Xeno, confronted, explains that Luna had been taken off the Hogwarts Express on the way back from school for Christmas break, to force Xeno to toe the party line in what The Quibbler was publishing; and that he had reported Harry's presence to the Ministry in the hope that if they had Harry, they might release Luna.

When Harry, Hermione, Ron, Dean Thomas, and Griphook are captured by Snatchers, they are taken to Voldemort's headquarters in Malfoy Manor. Harry, Ron, Dean, and Griphook are imprisoned in a basement room, and they find that Luna and Mr. Ollivander are already there. Luna provides a sharpened bit of metal with which she is able to cut Harry's and Ron's bonds. Harry, seeing a sky-blue eye in the shard of Magic Mirror he still carries with him, calls for help. Shortly, Dobby appears, and Apparates Luna, Dean and Mr. Ollivander to Shell Cottage, the home of Bill and Fleur. She says a few words at Dobby's funeral.

Luna appears back at Hogwarts shortly after Harry does; Neville has used the Galleons that Hermione had created to announce meetings of Dumbledore's Army to summon the DA members to Hogwarts. She accompanies Harry to Ravenclaw tower, and shows him the bust of Ravenclaw with the model of the lost diadem. When Alecto Carrow surprises Harry and has him at wand point, Luna Stuns her. She admits that she is surprised at how loud it is.

Luna, along with Ginny and Hermione, are battling Bellatrix in the renewed battle in the Great Hall, when Molly Weasley, enraged at the sight of a Killing curse that barely misses Ginny, attacks Bellatrix.

At the victory celebration, Harry is sitting beside Luna. She recognizes that he feels very tired, and suggests that he should leave and get some rest; she calls out and points out the window, and as everyone nearby is distracted by this, Harry slips on the Invisibility Cloak and departs, stopping only to gather Hermione and Ron as he does so.

Even though Luna is seen as an odd and weird person, she is quite clever. Possibly due to her odd family life, being raised by an eccentric like Xeno Lovegood, she sees things in different lights, and thus sees things that other people probably wouldn't. It is true that she believes in creatures, and magical properties of creatures and plants, that are not commonly accepted in the Wizarding world, but this could be seen as simply a mind that is a little more open than usual. She is unusually perceptive about the people around her, such as when Luna immediately recognizes the disguised Harry at the wedding in "Deathly Hallows" just by the expression under his polyjuiced face. She is also kindhearted and compassionate, and good at comforting others, showing these qualities to Harry, Ron, Hermione, Neville and Mr Ollivander.

Despite being teased and having her possessions stolen by classmates, Luna feels little animosity towards them. Luna seldom gets angry. In tense or dangerous situations Luna is much calmer than the people around her.

Luna tends to "space-out" at times, making her somewhat oblivious to her surroundings. Her belief in plants, creatures, and other things outside the usual magical community's belief structure may be the sign of a mind that is a little too open, and acts to isolate her from her classmates. She has a tendency to be over-honest in her assessments of others, a trait which Harry finds somewhat dismaying at times.

Commonly referred to as "Loony" Lovegood, Luna is often alone, but she is not a loner. Her eccentric behavior and appearance have given her an unlucky reputation at Hogwarts that has led to practical jokes and unpleasant whisperings. Despite this, Luna remains friendly and fairly optimistic.

Luna is treated as an oddball and outsider by her Ravenclaw housemates. She tells Harry that she is lonely without D.A. meetings in her fifth year implying that she has no friends in her own House. All those she considers to be her true friends are Gryffindors.

Perhaps because of her tendency toward introversion, Luna is one of the most observant characters in the series. She is known to voice even the most unpleasant of truths about other characters; notably, she comments to Harry in Half-Blood Prince that Ron can be quite cruel at times. However, her observations are never made with malice or contempt – she merely states the facts as she sees them.

Luna's friendships are few and far between. As a member of the DA, she is commonly seen with Neville Longbottom. In Half-Blood Prince, Harry invites Luna to accompany him to a party as "just friends," at which she is ebullient. Visiting her house in "Deathly Hallows" Harry finds a painting she made with the faces of Harry, Hermione, Ron, Neville and Ginny linked together with the word "friends" over and over. These friendships are very important to her.

Luna Lovegood is portrayed as being somewhat odd in her behavior. We hear that she is nicknamed "Loony", and note the contradiction of her apparent detachment from ordinary life and continued belief in odd creatures, contrasted with her awareness of character and level-headed dealing with adversity. On first meeting her, Harry notes that she seems somewhat dreamy, detached from the passing scene, an observation heightened by the earrings that seem to be made out of radishes. (It is only in the last book that we learn that these are so-called "dirigible plums", which her father Xenophilius Lovegood believes heighten mental acuity.) It is in that first meeting also that Harry learns of Luna's belief in creatures that few others believe exist, a belief that comes from Xeno's teaching. It is some time later that Luna observes that Ron can say things that are quite hurtful even if he doesn't mean to be. In the final book, where we see Luna captive in the cellars of Malfoy Manor, we learn that she has managed to find a tool with which she was able to free herself and Mr. Ollivander from their bonds; in her dealings with Harry at that juncture, she seems quite calm and collected. We further note that in the DA, she is one of the students who is able to produce a corporeal Patronus, implying significant magical ability.

We cannot be certain, but the portrayal of Luna strongly suggests that mentally she falls somewhere on the autism spectrum. People with autism tend to have difficulty making friends, and we see that Luna seems to consider the Trio as her primary friends despite having only met them in her fourth year at Hogwarts. Those who manage to deal with the disorder often have a tendency towards introspection, and the understanding of themselves does sometimes allow more of a conscious understanding of others and the ways they behave. People with autism also have a tendency to focus on one topic at a time, which allows them to gain an encyclopedic understanding of that topic; we see some of that tendency in Luna. And in small details, like the precision with which she answers the eagle's question to enter the Ravenclaw common room in the seventh book, we see the sort of attention to minutiae that is common in people with mild autism.

We suspect that the author's intent in introducing Luna may have been in part an attempt to introduce other-mindedness to a young audience. Luna is certainly one who thinks differently from most, and is unafraid to display that difference. It is possible that she is as intelligent as Hermione, but that is not obvious, as Hermione is written as something of a show-off, at least initially, and never attempts to hide her knowledge, while Luna, alongside her unusual belief structure, seems to be keeping her understanding to herself to avoid having to interact with her fellow students. The author may have intended to illustrate that it is not necessary to be visibly bright to be intelligent, and that those who think in a different manner are still valuable people and worth getting to know.

A small set of series fans suggested at one point that Luna would be Harry's eventual romantic interest. It was suggested that Harry, damaged by having to live with the Dursley family, and Luna, with her mental differences, might settle down to heal each other. While we are unaware at this point of any "fanfic" that puts them together as a couple, we would not be surprised to find that such exists.